Although Ca2+ is critically important in activity-dependent neuronal development not much is known about the regulation of dendritic Ca2+ signals in developing neurons. of ～160?nM seen in animals aged 4?weeks. Although the AP-evoked dendritic Ca2+ load increased about four times during the first 4?weeks the peak amplitude of free Ca2+ concentration was balanced by a four-fold increase in Ca2+ buffer capacity κs (～70 was estimated by calculating the ratio of the peak BI207127 amplitude of the AP-induced reduction of fluorescence in the background ROI and the dendritic ROI. As the BI207127 fluorescence of our ROIs is the sum of a Ca2+-dependent Fura signal and a Ca2+-independent background signal from (1) and (2) resulted in an estimate of the Fura-independent background signal fluorescence traces were subtracted from the fluorescence signals according to eqn (1) to finally BI207127 obtain the dendritic Fura fluorescence fluorescence signals according to eqn (1). For the isosbestic excitation wavelength eqn (2) was subtracted from eqn (1) to calculate the background-corrected and rapidly bind and distribute between the different buffers. The total amount of Ca2+ ions (Ca2+tot) will partially increase the Ca2+-bound RHPN1 fraction of fura (Δ[BCa]) the Ca2+-bound fraction of the endogenous Ca2+ buffers (Δ[SCa]) and will also appear as an increase in the free Ca2+ concentration (Δ[Ca2+]i). This relationship can be expressed as: (7) with representing the total increase of the intracellular Ca2+ concentration. Therefore the increase in the intracellular Ca2+ concentration is linearly related to the Ca2+ binding ratio of fura-2: (8) After the rapid increase the Ca2+ transient exponentially decays back to resting Ca2+ concentration according to the equation: (9) BI207127 with the peak amplitude APs with an interspike interval APs (3-10 APs at 100?Hz) were repeated 10 times at a rate of 5?Hz. To simulate Ca2+ accumulation during TBS firing we first calculated the integral of the Ca2+ signals as a linear summation of single AP-evoked transients from eqn (9): (14) As a second approach we modelled TBS more precisely taking into account that BI207127 the Ca2+ influx and therefore the slope of rise of the individual burst-evoked Ca2+ transients decreased during TBS. This was represented by an experimentally obtained normalized rise-time factor towards the decay time constant measured after the last burst τ10. Thus the integral of the Ca2+ signal can be expressed as: (15) with and and and and and and and and and and and and and and and and D the simulations (solid lines) nicely fit the measured data. This analysis shows that the pronounced activity-dependent slow-down of Ca2+ extrusion in young pyramidal cells is sufficient to explain the remarkable supralinear increase in the dendritic Ca2+ concentration and the total integral BI207127 of the Ca2+ signal during theta-burst firing. Discussion The analysis of AP-evoked dendritic Ca2+ transients in rat CA1 pyramidal cells during the first 4?weeks of postnatal development revealed several unexpected results. The first surprising finding was that AP-induced dendritic Ca2+ transients rapidly increase after P5 and achieve an amplitude of ～145?nm per AP already 1?week after birth (P7-P9) similar to the amplitude measured in cells from 4-week-old rats (～160?nm per AP). Secondly we were able to show that these similar amplitudes are generated by the balanced upregulation of both the total dendritic calcium load per AP (from ～10?μm to ～40?μm) and the endogenous Ca2+ buffer capacity (from ～70 to ～280) (Table?(Table1).1). Furthermore the Ca2+ extrusion after the APs was about five times slower in cells at 1?week than at 4?weeks resulting in a slower decay in young (τ?=?0.17?s) than in mature (τ?=?0.09?s) cells and a more effective temporal summation during brief bursts of APs. Finally during continuous theta-burst firing dendritic Ca2+ concentration was up to three-fold larger 1?week after birth than in 4-week-old animals. We show that this reflects an activity-dependent slow-down of Ca2+ extrusion and a resulting supralinear temporal summation of AP-evoked Ca2+ signals specifically in young pyramidal cells. Postnatal development of Ca2+ influx and buffering The amplitude of our AP-evoked dendritic Ca2+ transients (～150?nm) and the decay time course of τ?≈ 0.1?s in cells from animals aged 2?4?weeks are consistent with several previously published results (Helmchen et?al. 1996; Maravall et?al. 2000). However the data obtained with younger animals (P7?P9 pyramidal cells) contrast strongly with findings in previous publications showing calcium transients per AP.